The present invention relates to a semiconductor device, and more particularly, to a device for discharging static electricity, which can protect an internal circuit against an electrostatic current.
Generally, electrostatic discharge (ESD) is a rapid flow of electric current that takes place when two mutually insulated objects with significantly different potentials come into direct contact with each other.
When an ESD is introduced to a semiconductor device, circuits inside the device can be damaged. In order to protect internal circuits, most semiconductor devices include one or more ESD protection units between an externally located electrical contact pad for the device and internal circuits.
A diode, a metal oxide silicon (MOS) transistor and a silicon controlled rectifier (SCR) are all used as ESD protection units.
An SCR is an ESD protection unit, which can receive a high ESD current per unit area of its semiconductor junctions and which has low junction capacitance and low operating resistance. However, an SCR disadvantageously has a higher operation voltage than other ESD protection units.
FIG. 1 illustrates a conventional prior art circuit 10 for discharging static electricity.
Referring to FIG. 1, in order to discharge an electrostatic current introduced to an input/output pad 102 to a ground voltage line (or VSS: 104), the conventional circuit 10 for discharging static electricity uses a silicon controlled rectifier (SCR) having a relatively high operation voltage as a discharge part between the input/output pad 102 and the VSS 104. In order to reduce the operating voltage of the SCR 110, the circuit 10 also includes an NMOS transistor 120 as a drive element, connected between the input/output pat 102 and the SCR 110, and which detects a drive current 12 and then supplies it to the SCR 110. Therefore, the device 10 operating voltage can be reduced to a breakdown voltage of the NMOS transistor 120, while ESD effect of the SCR 110 is maintained.
In the device shown in FIG. 1, the SCR 110 is not operated until the electrostatic voltage reaches the brakedown voltage. Thus, a circuit internal to a semiconductor device might not be adequately protected from static electricity at an initial or relatively low voltage range of an electrostatic pulse.